It is a well known fact that it is very difficult, or even impossible, to fabricate capacitors which have exactly the desired capacitance, particularly in the case of parallel-plate capacitor arrangements where the overlapping area between a first electrode means and a second electrode means gives the capacitance due to allowable misalignment during the fabrication process. Thus, it is not possible to guarantee that the capacitance will be exactly the desired one; it is only possible to guarantee that it will fall within a range given by an allowable misalignment, i.e. an allowable misalignment limit. This is clearly disadvantageous since in many cases an exactly defined capacitance is needed. A particular case relates to so called varactors, i.e. tunable capacitors. Coplanar plate and parallel plate electrodes are for example considered for applications in phase and frequency agile, i.e. tunable, adaptable, reconfigurable, microwave systems. In comparison with analogue (semiconductor, MEM) varactors, the varactors based on the use of a ferroelectric film have a higher tuning speed, a higher Q(quality)-factor and lower leakage currents, which is very advantageous.
FIG. 1A shows very schematically a coplanar plate varactor arrangement 1001 wherein two coplanar electrodes 301, 301′ are deposited on top of a ferroelectric film 201 which in turn is disposed on a substrate 101. For the varactor arrangement 1001 of FIG. 1A, for a given ferroelectric film, the capacitance is defined by the shape of the electrodes and the gap width g between the electrodes 301, 301′. In parallel-plate varactors as shown in FIGS. 1B-1F, the ferroelectric film is instead sandwiched between two electrodes, c.f. FIG. 1B wherein a ferroelectric film 202 is disposed between a top electrode 302′ and a bottom electrode 302 which is disposed on a substrate 102. For a given ferroelectric film, the capacitance is defined by the thickness t of the ferroelectric film in the area where the top and the bottom electrodes overlap and by the overlapping area and by the dielectric permittivity of the film.
FIG. 1C shows an alternative implementation of a parallel-plate varactor arrangement wherein a top electrode 303′ is disposed on a ferroelectric film 203 partly in overlap with a bottom electrode 303 disposed on a substrate 103. The varactor arrangement 1003 of FIG. 1C has a capacitance which, for a given ferroelectric film, is given by the overlapping area which is given by the width w×l+Δl, wherein w is the width of the overlapping portion and l+Δl is the length of the overlapping portion.
FIG. 1D shows still another known parallel-plate varactor arrangement 1004 comprising a substrate 104, a top electrode 304′ and a bottom electrode 304 which are disposed on either sides of a ferroelectric film 204 such as to partly overlap. A low permittivity film (with dielectric constant ∈<10) 404 is arranged such as to define the overlapping area, the length portion b where there is no such extra film defining the actual relevant portion. A top view of this arrangement is shown in FIG. 1E where it can be seen the width c of the overlapping portion and hence the overlapping area being defined by the opening b×c in the low permittivity film.
In still another parallel-plate arrangement 1005 comprising a substrate 105, dielectric film 205 lower and upper electrodes 305, 305′ an opening is formed in the bottom electrode (305) or alternatively in the top electrode (305) to define an overlapping area A=b×c.
However, all these known varactor arrangements suffer from drawbacks. For example, varactors with coplanar plate electrodes as shown in FIG. 1A have a simple design but they require application of higher voltages than parallel-plate varactor arrangements, typically the required voltage is above 50-100V. Varactors with parallel-plate electrodes as shown in FIG. 1B-1C do not require such high voltages but typically it is enough with a voltage of 5-20V, but on the other hand it is a disadvantage of such designs that they are sensitive to the alignment of the top and the bottom electrodes during the fabrication process. Normally a ferroelectric film with an extremely high permittivity is used and due to this extremely high permittivity, which typically is above 100, a small disalignment Δl (c.f. FIG. 1C) will result in substantial changes in the capacitance, which make the prediction of the capacitance non-controllable and hence the design of the arrangement will not be cost-effective. The design shown in FIG. 1D, 1E offers a good capacitance prediction but it requires more masks and fabrication processes making them cost ineffective. The arrangement shown in FIG. 1F offers a comparatively good capacitance prediction but it is disadvantageous in so far that extra ohmic losses are associated with strips connecting the leads or pads of the capacitor of the overlapping area of the parallel-plate structure.